1. Field of the Invention
The present invention relates to a system and a method for ceramic crystallization and single crystal growth, particularly designed for the material systems with compositional segregation during solidification. More specifically, this invention relates to (i) crystal growth systems and methods derived in part from a vertical Bridgman approach for volatilizing ternary and binary solid solutions including lead-contained ternary or binary solid solutions, including but not limited for PMN-PT based crystals (lead magnesium niobate-lead titanate solid solutions); (ii) a batch auto-feeding system and mechanism with high temperature batch flow control operation and (iii) an automation control system for operably controlling a crystal growth processes. The proposed system and methods are This invented crystal growth method is particularly used for growing crystals of lead-contained binary and ternary solid solutions to minimize the compositional segregation effect.
2. Description of the Related Art
Acoustic transducers are the operational center of many medical, commercial, and military imaging systems. The most common types of transducers utilize lead zirconate titanate (PZT) based ceramics as a piezoelectric function. Piezoelectric ceramics convert mechanical energy into electrical energy and conversely electrical energy into mechanical energy. While conventional PZT materials remain the most common materials used in acoustic transduction devices, changing material requirements have fostered the need for new piezoelectric materials having improved dielectric, piezoelectric and mechanical properties.
Single crystals of solid solutions of Pb(Mg1/3Nb2/3)O3 (PMN) and Pb(Zn1/3Nb2/3)O3 (PZN) with PbTiO3 (PT) (PMN-PT and PZN-PT) have generally desirable ultrahigh piezoelectric properties, coupling constants (k33), and dielectric constants that are unachievable in conventional piezoelectric (PZT) ceramics.
At ambient temperatures, the morphotropic phase boundary (MPB), separating rhombohedral phase from tetragonal phase, exists in (1-x)PMN-xPT system at about x=0.34, and in (1-x)PZN-xPT system at about x=0.09. The crystals of compositions close to the MPB, the so-called relaxor-based single crystals, have shown greatly desired piezoelectric properties suitable for use in medical imaging devices. Unfortunately, the electromechanical properties of these types of single crystals are very sensitive to the orientation and chemical composition of the crystal (See for example U.S. Pat. No. 6,465,937 issued Oct. 10, 2002, the entire contents of which are herein incorporated by reference), and have been very hard if not impossible to produce in commercial, reliable, and homogenous quantities.
In early 1980s, Kuwata et al. (J. Kumata, K. Uchino and S. Nomura, Dielectric and piezoelectric properties of 0.91Pb(Zn1/3Nb2/3)O3-0.09PbTiO3, Jpn. J. Appl. Phys., 21, 1298-1302 (1982)) found very high piezoelectric coefficient, d33, of 1500 pC/N and electromechanical coupling factor, k33, of 0.92 in 0.91PZN-0.09PT single crystals along <001> direction.
Later, high properties were also observed in PMN-PT crystals by Shrout and his co-workers in 1990 (T. R. Shrout, Z. P. Chang, N. Kim and S. Markgraf, Dielectric behavior of single crystals near the (1-x) Pb(Mg1/3Nb2/3)O3-xPbTiO3 Morphotropic Phase Boundary, Ferroelectrics Lett., 12, 63-69 (1990)).
High electromechanical coupling (k33)>90%, piezoelectric coefficient (d33)>2500 pC/N and large strain up to 1.7% were reproducibly observed in the later 1990's (S. E. Park and T. R. Shrout, Ultrahigh strain and piezoelectric behavior in relaxor based ferroelectric single crystals, J. Appl. Phys., 82, 1804-11 (1997)).
The super-high piezoelectric properties noted in this literature promised a new generation of acoustic transduction devices but unfortunately were highly difficult to manufacture using any known method.
The small single crystals of PMN-PT and PZN-PT discovered above were obtained by a conventional flux growth method. Unfortunately, usefully sized single crystals (at least inch size) of good quality were long unavailable until in 1997 when PZN-PT single crystals were grown by improved flux growth methods. See S. E. Park and T. R. Shrout, Characteristics Of Relaxor-Based Piezoelectric Single Crystal For Ultrasonic Transducers, IEEE Trans. On Ultrasonics, Ferroelectrics and Frequency Control, Vol. 44, No. 5, 1140-1147 (1997); and T. Kobayashi, S. Shimanuki, S. Saitoh, and Y. Yamashita, Improved Growth Of Large Lead Zinc Niobate Titanate Piezoelectric Single Crystals For Medical Ultrasonic Transducers, Jpn. J. Appl. Phys., 36, 6035-38 (1997).
A conventional Bridgman method (P. W. Bridgman, Proc. Am. Acad. Sci. 60 9 (1925) 303) is characterized by a relative translation of a crucible containing a melt along a single axial temperature gradient in a vertical furnace. A Stockbarger method (D. C. Stockbarger, Ref. Sci. Instrum. 7 (1963) 133) is a modification of the Bridgman method and employs a single heat insulation buffer separating a vertical furnace into only two zones, a high temperature zone and an upper low-temperature zone.
Recently, a modified vertical Bridgman growth method was developed for large sized crystals: PZN-PT single crystals associated with flux (Y. Hosono, K. Harada, S. Shimanuki, S. Saitoh, and Y. Yamashita, Crystal Growth And Mechanical Properties Of Pb(Zn1/3Nb2/3)O3-Pbtio3 Single Crystal Produced By Solution Bridgman Method, Jpn. J. Appl. Phys., 38, 5512-15 (1999)) and PMN-PT single crystals using a crucible moving-downward method in a temperature gradient (Chinese Pat. No. CN 1227286A, “Method Of Preparation Of Relaxor Ferroelectric Single Crystal Lead Magnesium Niobate-Lead Titanate” by H. Luo et al., published Sep. 1, 1999 and H. Luo, G. Xu, H. Xu, P. Wang, and Z. Yin, Compositional Homogeneity And Electrical Properties Of Lead Magnesium Niobate Titanate Single Crystals Grown By A Modified Bridgman Technique, Jpn. J. Appl. Phys., 39, 5581-85 (2000).
Unfortunately, substantial challenges still exist in manufacturing piezoelectric single crystals. One challenge is that a lead-contained melt, at high temperature, is made highly toxic through the evaporation (volatilization) of lead oxide and corresponding and detrimental increases compositional segregation due to the loss of critical elements. This challenge alone eliminates commercially viable and available crystal growth techniques. Further, the electromechanical properties of the relaxor-based PMN-PT crystals with 25˜35% PT contents close to the MPB are critically sensitive to the PT content and the evaporation generates wildly variable electromechanical properties. An additional challenge is that crystal growth with flux association yields a very low growth rate and unacceptable imperfection manifestations, including micro inclusions drastically dropping yield. Finally, each of these methods provides poor homogeneity and greatly reduced material utilization factors raising production costs (material losses).
It is also clear that the Bridgman-type growth method alone is only feasible for PMN-PT crystal due to the pseudo-congruent behavior of the binary solid solution system. So far no publications gave the reason for this behavior and there is no calculable way to predict it due to the absence of most important of the thermodynamic parameters. (Only the experimental results, presented herein indicate the crystallization behavior.)
Referring now to the Bridgman growth method discussed above, this method allows for PMN-PT crystal growth at relatively fast rates, up to 1 mm/hr, but the resultant compositional segregation is detrimentally large generating tremendous yield loss via PT loss. The PT variability provides unpredictable and undesirable piezoelectric properties reducing material utilization to a vary small range. The resultant compositional segregations prevent commercial implementation of rapid growth rates without unacceptably high quality control losses.
Referring now to FIG. 1, a conventional single PMN-PT crystals was grown using solely the Bridgman growth method and the results are noted in the accompanying graph. Clearly the segregation results using this conventional growth method and system resulted in great Pb, Mg, Nb, and Ti variability along the length of the grown boule. The growth parameters were: seeding [110], growth rate 0.8 mm/hr at temperature gradient 30° C./cm, and maximum crucible temperature of 1365° C. The Induction Coupled Plasma (ICP) spectroscopy employed had accuracy greater than 0.5%. It is clear from the figure that there is wide compositional variability along the length of the boule. This variability is drastically significant even within 1 cm length increments effectively rendering the entire boule unusable. It is clear that this method of crystal growth is incapable of providing useful lengths of compositionally homogenous material.
As noted above, since the piezoelectric properties of PMN-PT single crystals are sharply dependant upon composition, this composition variability results in a great reduction of the useful portion of the as-grown crystal boule, and increased production, handling, and testing costs. As seen in the FIG. 1, the percentage % of each of the compositional changes is as much as 10-25% within 1 cm length. This variability is unacceptable for commercial implementation.
Consequently a long felt need existed for an improved approach and Applicant has developed a hybrid method: Zone-leveling Bridgman approach to manufacture large-sized PMN-PT crystals with greatly improved compositional homogeneity (See U.S. Pat. No. 6,942,730, the entire contents of which are herein incorporated by reference). However, upon further practice of Applicant's patented invention, additional limitations were realized. Consequently Applicant's '730 invention is now recognized as only partially solving the segregation problem due to the limitation of size of the melting zone for large diameter crystals and other factors discussed below.
While it was suggested that a continuous feed of raw materials may assist the compositional change taking place in the liquid phase the continuous feed method is suitable only to Mn—Zn Ferrite that has an extremely low melting point and less or no volatility when compared to Pb-contained PMN-PT compound. (See the book chapter “Crystals for magnetic applications” in series of “Crystals growth, properties and applications”, edited by C. J. M. Rooijmans, Springer-Verlag, Berlin Heidelberg, 1978. Consequently, the related art has failed to appreciate the need for a system and method that overcomes the detriments noted above.
In summary, the problems of commercially available manufacturing methods for PMN-PT based single crystals include at least the following:                1. low unit yield and high manufacturing cost due to loss in the process.        2. gross compositional inhomogeneity resulting in variation of piezoelectric properties rendering the resultant product wholly unsuitable for commercial use.        
Accordingly there is a need for an improved crystal growth system and method that addresses the commercial needs discussed. Additionally, and alternatively, there is a need for a composition that operates with the proposed system and method.